When exposed to radiation such as X-rays, an energy-storing phosphor (e.g., stimulable phosphor, which gives off stimulated emission) absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the panel; sequentially scanning the storage panel with stimulating light such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and a phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer so as to keep the phosphor layer from chemical deterioration or physical damage.
Various kinds of energy-storing phosphor layers are known. For example, the phosphor layer can comprise a binder and energy-storing phosphor particles dispersed therein, or otherwise can comprise agglomerate of an energy-storing phosphor without binder. The latter layer can be formed by a gas phase-accumulation method or by a firing method.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. However, it is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of as high quality (in regard to sharpness and graininess) as possible.
JP-A-2004-132872 discloses a radiation image storage panel containing a pigment in the form of particles having a mean size of 0.1 to 1.2 μm. The pigment shows integrated absorbances (a) and (b) at the wavelength regions of 670 to 690 nm and 390 to 420 nm, respectively, under the condition of 0.80≦a/b≦3.00.
JP-A-2004-177314 discloses another radiation image storage panel comprising a stimulable phosphor layer. The phosphor layer contains stimulable phosphor particles having a mean size of 3.0 μm or less and, in addition, pigment particles having a mean size of 1.2 μm or less in an amount of 0.0002 to 0.001 wt. % based on the amount of the stimulable phosphor. Both radiation image storage panels disclosed in the above-identified publications have object for improving quality, particularly, sharpness of the reproduced radiation image.
As described above, for the purpose of improving the sharpness of radiation image, it has been proposed and practically adopted to incorporate a colorant such as pigment into the phosphor layer of the radiation image storage panel. The radiation image storage panel capable of giving a radiation image of high sharpness is practically used in precise examination such as dental diagnosis and non-destructive inspection, as well as in the conventional radiography. In the precise examination, the storage panel is required to give a radiation image having both high sharpness and no imaging defects such as arcifacts. Even if there are some scratches on the panel surface, it is necessary for the scratches not to cause arcifacts on the image. In other words, the radiation image storage panel must be excellent in resistance to scratch Accordingly, it is desired to provide a radiation image storage panel not only giving high sharpness but also having excellent scratch resistance.
However, it is difficult to manufacture a radiation image storage panel having both excellent scratch resistance and high sharpness at the same time. When the surface of the storage panel is scratched, the phosphor layer in the scratched area is locally dented. Since the phosphor layer is thus rendered to have uneven thickness, arcifacts are liable to appear on the resultant reproduced image. For the reason, the higher sharpness the radiation image storage panel gives, the more likely the resultant reproduced image suffers arcifacts.